Circuit breakers are designed to open their contacts either upon manual operation or upon the occurrence of an overcurrent. Contacts occasionally refuse to open as expected and lock in the closed position. Certain standards applicable to circuit breakers sold in some countries require that in the event of locked contacts, the operating handle should generally indicate ON. Although locked contacts are a relatively rare occurrence, they create a potentially hazardous situation because the circuit breaker has operated in an unexpected manner. Current is flowing through at least one phase of the circuit breaker when the operator or maintenance personnel has reason to expect that the circuit breaker has opened and has interrupted the current. Maintenance personnel May check the circuit breaker handle and find that it indicates open contacts and then begin working on nearby electrical equipment or on the circuit breaker itself.
In the prior art this problem was remedied by a circuit breaker mechanism, which upon the contacts being locked together, could not be moved from the ON position or moved back to the ON position after being manually moved away. The operating handles of these designs indicate when there is current flowing through the circuit breaker. However, these designs do not alert personnel to the fact that the contacts are locked. Maintenance personnel may only know that the contacts are closed and will expect the circuit breaker to interrupt the current should a fault later occur.